1. Field of the Invention
The present invention relates to an optical disk used for recording/reproducing information and a method for producing the same.
2. Description of the Related Art
In the field of magneto-optical disks, there is a demand for high-density recording of information. As a method for realizing such high-density recording, a domain wall displacement detection (DWDD) system has been proposed.
In an optical disk of the DWDD system, it is required to weaken magnetic coupling between adjacent recording tracks. Therefore, in the case of producing an optical disk of the DWDD system, initialization for weakening magnetic coupling between adjacent recording tracks is conducted before recording an information signal. Such an initialization method has been reported in the past (see JP 6(1994)-290496 A and JP10(1998)-340493 A).
FIG. 10 shows an example of a structure and an initialization method of a conventional optical disk. As shown in FIG. 10, a conventional optical disk 1 includes a substrate 2, and a first dielectric layer 3, a recording layer 4, a second dielectric layer 5, and a protective coating layer 6 successively stacked on the substrate 2. On the surface of the substrate 2 on the side of the recording layer 4, grooves 2a are formed. A portion called a land is formed between two grooves 2a adjacent in the radial direction, and this portion becomes a recording track. The groove 2a has a width of, for example, 0.2 xcexcm, and the land has a width of, for example, 1.4 xcexcm. The recording layer 4 includes at least three magnetic layers for reproducing information by the DWDD system.
Next, an initialization method of the optical disk 1 will be described. According to the initialization method of the optical disk 1, laser light 7 (laser power=10 mW, xcex=780 nm, NA of an objective lens 8=0.5, diameter of a light spot=about 800 nm) for annealing is radiated along the grooves 2a, thereby eliminating magnetic coupling of the recording layer 4 on the grooves 2a. During the initialization process, the relative moving speed of the light spot of the laser light 7 is, for example, 2 m/sec.
However, according to the initialization method as described above, portions other than the grooves 2a also are irradiated with a light spot. Therefore, an effective recording track becomes narrow, which decreases a signal level. This requires a light spot radiated to the recording layer 4 to be small. The first dielectric layer 3 is optimized in accordance with a wavelength of laser light for recording/reproducing, so that it is difficult to decrease the size of a light spot of the laser light 7 for annealing. Furthermore, for the same reason, it is difficult to enhance an absorption efficiency of the laser light 7 for annealing in the recording layer 4, so that initialization at a high linear velocity cannot be conducted, resulting in a long initialization time.
Therefore, with the foregoing in mind, it is an object of the present invention to provide an optical disk having a high recording density, which can be initialized in a short period of time, and a method for producing the same.
In order to achieve the above-mentioned object, there is provided a method for producing an optical disk including a substrate and a recording layer disposed above the substrate and reproducing an information signal by a DWDD system, using light incident from the substrate side, the method including the processes of: (i) forming a first dielectric layer, the recording layer, and a second dielectric layer on the substrate in this order; and (ii) irradiating the recording layer with laser light for initialization from the second dielectric layer side, thereby weakening magnetic coupling of a part of the recording layer. According to the above-mentioned method for producing an optical disk, an optical disk can be produced with a high recording density and satisfactory productivity. In the present specification, the term xe2x80x9cinitializationxe2x80x9d refers to a process of weakening magnetic coupling between recording tracks by annealing a part of the recording layer.
In the above-mentioned production method, it is preferable that a wavelength of the laser light for initialization is xcex, and a thickness of the second dielectric layer is in a range of xcex/(12xc3x97n) to xcex/(2xc3x97n) (where n is a refractive index of the second dielectric layer), particularly, in the vicinity of xcex/(4xc3x97n).
In the above-mentioned production method, the laser light for initialization may be obtained by condensing laser light by an objective lens with a numerical aperture of at least 0.65. According to this constitution, a laser spot can be made small, and an optical disk with a particularly high recording density can be produced.
In the above-mentioned production method, during the process (ii), the recording layer may be irradiated with laser light for tracking servo, whereby tracking servo is conducted. According to this constitution, the precision of tracking control is enhanced, so that positional precision in the radial direction of annealing also is enhanced, resulting in a stable DWDD operation.
In the above-mentioned production method, a wavelength of the laser light for initialization may be shorter than a wavelength of the laser light for tracking servo. According to this constitution, track density can be enhanced.
The above-mentioned production method further may include forming a heat conduction adjusting layer for adjusting the sensitivity of the recording layer on the second dielectric layer after the process (ii).
In the above-mentioned production method, during the process (ii), a region of the recording layer to be irradiated with the laser light for initialization may be heated before being irradiated with the laser light for initialization. According to this constitution, a time required for initialization can be shortened, and an optical disk can be produced with satisfactory productivity.
Furthermore, a first optical disk of the present invention includes a substrate and a recording layer disposed above the substrate, and reproduces an information signal by a DWDD system, using light incident from the substrate side, the optical disk further including a first dielectric layer disposed between the substrate and the recording layer and a second dielectric layer disposed on the recording layer opposite to the substrate, wherein magnetic coupling of a part of the recording layer is weakened by irradiation with light having a wavelength xcex incident from the second dielectric layer side, and a thickness of the second dielectric layer is in a range of xcex/(12xc3x97n) to xcex/(2xc3x97n), where n is a refractive index of the second dielectric layer. In the above-mentioned optical disk, a thickness of the second dielectric layer is stipulated in accordance with laser light for initialization. Therefore, a spot of laser light for initialization can be made small, and the annealing efficiency by laser light for initialization can be enhanced. Thus, according to the above-mentioned constitution, an optical disk with a high recording density is obtained that can be initialized in a short period of time.
In the above-mentioned first optical disk, the second dielectric layer may be made of silicon nitride and may have a thickness in a range of 40 nm to 60 nm, and the wavelength xcex may be in a range of 400 nm to 410 nm. According to this constitution, sufficient resistance to corrosion can be obtained. Furthermore, according to this constitution, the amount of light absorbed can be increased, and the energy of laser light required for initialization can be decreased.
In the above-mentioned first optical disk, the second dielectric layer may be made of silicon nitride and may have a thickness in a range of 25 nm to 30 nm, and the wavelength xcex may be in a range of 400 nm to 440 nm. According to this constitution, since the second dielectric layer is thin, an optical disk can be produced with satisfactory productivity.
In the above-mentioned first optical disk, a refractive index of the second dielectric layer may be larger than a refractive index of the first dielectric layer. According to this constitution, even the relatively thin second dielectric layer can utilize laser light for initialization efficiently, so that a time required for forming the second dielectric layer can be shortened.
The above-mentioned first optical disk further may include a heat conduction adjusting layer disposed on the second dielectric layer for adjusting the sensitivity of the recording layer. According to this constitution, the sensitivity of the recording layer can be adjusted, and a power range enabling information to be recorded/reproduced can be enlarged.
The above-mentioned first optical disk further may include a protective coating layer formed on the second dielectric layer opposite to the substrate, and the protective coating layer may be thinner than the substrate.
In the above-mentioned first optical disk, a difference in level may be formed on a surface of the substrate on the recording layer side, and recording tracks of the recording layer may be separated magnetically by the difference in level. According to this constitution, by irradiating lands with laser light for initialization, a region with a narrow width can be annealed using a light confinement effect. An optical disk is obtained in which magnetic coupling between recording tracks can be cut off easily, and a track pitch can be made narrow. Furthermore, by using grooves as recording tracks, portions with less surface roughness are used as recording tracks, whereby an optical disk with satisfactory DWDD characteristics can be obtained.
Furthermore, a second optical disk of the present invention includes a substrate and a recording layer disposed above the substrate, and reproduces an information signal by a DWDD system, using light incident on the substrate side, wherein sample servo pits for conducting tracking control by a sample servo system are formed on the substrate, grooves to be recording tracks are formed in a concentric shape or a spiral shape in a recording/reproducing region of the substrate, a track pitch of the recording tracks is in a range of 0.5 xcexcm to 0.6 xcexcm, and the recording track is cut off magnetically from an adjacent recording track by allowing a laser spot of laser light having a wavelength xcex in a range of 400 nm to 440 nm to scan an inter-groove portion.
The above-mentioned second optical disk may include a first dielectric layer disposed between the substrate and the recording layer, and a second dielectric layer disposed on the recording layer opposite to the substrate, wherein the recording track may be cut off magnetically from an adjacent recording track by irradiation with laser light from the second dielectric layer side.
In the above-mentioned second optical disk, a thickness of the second dielectric layer may be in a range of xcex/(12xc3x97n) to xcex/(2xc3x97n), where n is a refractive index of the second dielectric layer. In the second optical disk, information can be recorded with a high density. Furthermore, in the second optical disk, information can be reproduced with laser light having a wavelength in a range of 600 nm to 650 nm. A light source (laser diode) of laser light in such a wavelength range can be obtained easily, output a high power, and have stable characteristics.
In the above-mentioned second optical disk, a reflectivity RG of the groove and a reflectivity RL of the inter-groove portion may satisfy 0.95 less than RG/RLxe2x89xa61.0. According to this constitution, the movement of magnetic domain walls becomes smooth, and a signal ratio is enhanced.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.